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3281. Conversion from pair-like types to subrange is a silent semantic promotion

Section: 24.5.3 [range.subrange] Status: WP Submitter: Eric Niebler Opened: 2019-09-10 Last modified: 2020-09-06

Priority: 1

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Just because a pair is holding two iterators, it doesn't mean those two iterators denote a valid range. Implicitly converting such pair-like types to a subrange is dangerous and should be disallowed.

Suggested priority P1.

Implementation experience: range-v3's subrange lacks these constructors.

[2019-10 Priority set to 1 and status to LEWG after reflector discussion]

[2019-11 Status to Ready after LWG discussion Friday in Belfast.]

Proposed resolution:

This wording is relative to N4830.

  1. Modify 24.5.3 [range.subrange] as indicated:

    namespace std::ranges {
      template<class T, class U, class V>
        concept pair-like-convertible-to = // exposition only
          !range<T> && pair-like<remove_reference_t<T>> &&
          requires(T&& t) {
            { get<0>(std::forward<T>(t)) } -> convertible_to<U>;
            { get<1>(std::forward<T>(t)) } -> convertible_to<V>;
      template<input_or_output_iterator I, sentinel_for<I> S = I, subrange_kind K =
               sized_sentinel_for<S, I> ? subrange_kind::sized : subrange_kind::unsized>
        requires (K == subrange_kind::sized || !sized_sentinel_for<S, I>)
      class subrange : public view_interface<subrange<I, S, K>> {
        template<not-same-as<subrange> PairLike>
          requires pair-like-convertible-to<PairLike, I, S>
        constexpr subrange(PairLike&& r) requires (!StoreSize)
          : subrange{std::get<0>(std::forward<PairLike>(r)),
        template<pair-like-convertible-to<I, S> PairLike>
        constexpr subrange(PairLike&& r, make-unsigned-like-t(iter_difference_t<I>) n)
          requires (K == subrange_kind::sized)
          : subrange{std::get<0>(std::forward<PairLike>(r)),
                     std::get<1>(std::forward<PairLike>(r)), n}